The polysaccharide chitosan is the at least partially N-deacetylated derivative of chitin. Chitin can be found widely in the exoskeletons of arthropods, crustaceans and the cuticles of insects. It is usually derived from such natural sources. Chitosan in general is synthetically prepared by hydrolysis of chitin, although it can also be naturally derived directly, e.g. from certain fungi in which it occurs. The different solubilities of chitin and chitosan in dilute acids are commonly used to distinguish between the two polysaccharides. Chitosan, the soluble form, can have a degree of acetylation (DA) between 0% and about 60%, the upper limit depending on parameters such as processing conditions, molecular weight, and solvent characteristics. While soluble in acidic aqueous media, chitosan precipitates at a pH of above 6.3.
Both chitin and chitosan are promising polymers for biomedical applications because of their biocompatibility, biodegradability and structural similarity to the glycosaminoglycans. For comprehensive reviews of potential applications of chitin and chitosan see, e.g., Shigemasa and Minami, “Applications of chitin and chitosan for biomaterials”, Biotech. Genetic. Eng. Rev. 1996, 13, 383; Kumar, “A review of chitin and chitosan applications”, React. Funct. Polym. 2000, 46(1), 1; and Singh and Ray, “Biomedical applications of chitin, chitosan and their derivatives”, J. Macromol. Sci. 2000, C40(1), 69.
Aranaz et al. in “Functional characterization of chitin and chitosan”, Curr. Chem. Biol. 2009, 3, 203, discuss the antimicrobial activity of chitosan, including activity against bacteria, yeast, and fungi. A first mechanism discussed involves an interaction with the cell surface of gram-negative bacteria, which interaction is believed to prevent the transport of essential solutes. Another mechanism involves an inhibition of RNA and protein synthesis in the cell nucleus. This theory appears to predict that a relatively low molecular weight and a relatively low degree of acetylation should increase the chitosan's activity. It is, however, also pointed out that some authors have not found a clear relationship between the degree of acetylation and the antimicrobial activity of chitosan. E.g., in a study by Parker et al. 25% acetylated chitosan showed more effective antimicrobial activity compared with that of 10% and 50% acetylated chitosan. Other suggested mechanisms involve the activity of chitosan as a chelating agent, chitosan's activity to interact with flocculate proteins, and a direct disturbance of membrane function in fungi.
The patent application WO 2010/021930 A1 discloses activity of several chitosan derivatives against bacteria including methicillin-resistant Staphylococcus aureus. 
The patent application US 2009/0117213 A1 discloses a chitosan/alcohol solution that has antiviral, antibacterial and hemostatic effects. In particular, a solution comprising 1.5% chitosan with a molecular weight between 150 and 300 kD (kilodalton) and a degree of acetylation of 5%, the solution further comprising 25% v/v ethyl alcohol showed antibiotic activity in vitro against several strains of bacteria, including moderate activity against methicillin-resistant Staphylococcus aureus. 
Formulations of pharmaceutically or biologically active compounds are typically applied in a concentration which is higher than the concentration to be realized at the site of action, i.e. the effective or pharmaceutical concentration. This is due to the fact that once administered to an organism or any other site of application, e.g. an inanimate material, such formulations are typically subject of dilution before reaching the site of action.
Typical concentrations of pharmaceuticals at the biological site of action are considered less than μM concentrations. A 1 μM concentration of a molecule having a molecular weight of 100 D corresponds to a concentration of 0.0001 g/l or 0.000010% w/v (weight per volume). As a consequence, pharmaceutical compounds acting systemically and being associated with high bioavailability upon administration can be given as tablets of mg weights, such as corticosteroids. Such pharmaceutically or biologically active compounds are often and typically associated with adverse effects at higher concentrations. The window of therapeutically acceptable concentrations at the site of action is called therapeutic index. A number of compounds provide such a small therapeutic index that they need to be administered at almost pharmaceutical concentrations as infusion over time with immediate dilution upon infusion.
Some biologically active compounds are only active at concentrations which may not be practicable for a systemic application. An upper limit of practicability is given by compounds such as vitamin C or omega-3-unsaturated fatty acid which can be administered orally in gram amounts per individual. Saccharose with its potential to effectively block fungal or bacterial growth at concentrations of higher than about 30% can be used as a preservative in neutraceuticals or for stabilization of sensitive proteins, but is not suitable for the treatment of living material or organisms.
Chitosan requires a relatively high concentration at the site of action in order to exert a microbial effect but still provides a large therapeutic index. However, on the one hand, there is a common understanding in the prior art that the concentration of active ingredients in the composition to be applied non-systemically must be higher than required at the site of action due to dilution effects. Further, in view of that there are concerns that a poor solubility of chitosan in a pharmaceutically or biologically acceptable solvent may become a limiting factor for achieving a satisfactory microbial effect on or in the material to be treated. On the other hand, for therapeutic or prophylactic applications in humans the need for relatively high concentrations of chitosan at the site of action implies that introducing chitosan in the systemic circulation should be avoided.
Accordingly, there is a need for alternative compositions and methods for disinfecting or decontaminating materials harboring pathogenic microorganisms such as bacteria, fungi or viruses.